Rock wool for potting.

I haven’t seen it used for orchids outside of old books since I only started in the past couple of years, but I see no reason why it wouldn’t work, it’s just kind of hard to come by.

 

100% of my collection of paphs, phrags, and a few phals are in inert media, either LECA for those in semi-hydroponic culture, or a mixture of LECA and Grodan rockwool cubes (25%-33% of the volume) for those in "traditional" culture.

In the old days, I experimented with the rockwool batting that was sold in both hydrophobic and hydrophilic forms, and never had any success. Having discrete 1 cm cubes in an otherwise very open and airy mix works great. It's sort of an inorganic form of "bark with bits of sphagnum".

There is a great grower of phrags that has his in 75% cubes and 25% perlite - mainly to maintain air channels in the cubes, albeit smaller than I prefer.

(This seems like a natural evolutionary path - I started growing orchids while getting degrees in Ceramic Engineering in the 70's, so growing in clay and glass seems appropriate, even if it's taken me 50 years to get there.)